The “Coil-to-Globule” Transition of Poly(N-isopropylacrylamide) on the Surface of a Surfactant-Free Polystyrene Nanoparticle

Poly(N-isopropylacrylamide) (PNIPAM) adsorbed on surfactant-free polystyrene (PS) nanoparticles has been studied by a combination of static and dynamic laser light scattering (LLS). In static LLS, the amount of PNIPAM adsorbed on the nanoparticles was determined from the absolute excess scattered intensity; and in dynamic LLS, the temperature dependence of the hydrodynamic radius of the nanoparticles adsorbed with PNIPAM was monitored to reveal the “coil-to-globule” transition of PNIPAM on the particle surface. We found that the amount of PNIPAM adsorbed on the nanoparticles depends not only on the PNIPAM concentration but also on the highest temperature to which the nanoparticles/ PNIPAMmixture was heated. Near the lower critical solution temperature (LCST) of PNIPAM in water, the collapse of the adsorbed chains leads to an additional adsorption of PNIPAM on the surface. For a given temperature below the LCST, as the amount of the adsorbed PNIPAM increases, the thickness of the adsorbed PNIPAM layer increases, but the average density of the adsorbed PNIPAM layer decreases, suggesting an extension of the adsorbed chains. Moreover, our results indicate that the adsorbed chains have a lower LCST than the PNIPAM chains free in water.